In WO2014/182160 an offshore vessel with a wellbore drilling installation is disclosed which comprises:                drilling tower,        a drill floor having a well center through which a drilling tubulars string can pass along a firing line,        at least one vertical trolley rail supported by the drilling tower,        a trolley, said trolley being guided along said at least one vertical trolley rail,        a top drive device attached to the trolley, said top drive device comprising one or more top drive motors, e.g. electric top drive motors, adapted to impart rotary motion to a drilling tubulars string when connected to said top drive device,        a hoisting device adapted to move the trolley with the top drive device up and down along said at least one vertical trolley rails,        a vertical motion arm rail supported by the drilling tower,        a motion arm assembly comprising a base and an extensible and retractable motion arm, wherein the base is guided said at least one vertical motion arm rail, and wherein the motion arm has an operative reach that encompasses the firing line, said motion arm assembly being adapted to support at least one of a well center tool, e.g. an iron roughneck tool, or a tubular gripper member, and allowing to bring said well center tool or tubular gripper member in the firing line,        a vertical motion arm drive adapted to move the motion arm base along said vertical motion arm rail.        
In the field, especially in the offshore drilling field, downtime due to equipment failure is considered a major issue. Whilst a drilling or other wellbore related operation involves the use of numerous pieces of equipment, studies seem to indicate that the top drive device is one of the major contributors to undesirable downtime.
The company LeTourneau Technologies Drilling Systems Inc. identified the gearbox as crucial component in this regard and has developed a hollow shaft electric drive motor that acts as a direct drive for the rotary output stem or quill that is driven by the motor. This eliminates the gearbox yet requires a special design of the motor.
In US20130090200 the issue of top drive reliability is also addressed and an alternative structure of the transmission between the electric motors and the rotary quill is disclosed.
Other developments in the field concentrate on enhanced maintenance schedules for equipment, including of the top drive, to prevent failures from occurring.
Notwithstanding the efforts made so far to reduce downtime, failures of top drive devices still occur at the expense of very costly downtime of the drilling installation.